Stainless steel

ABSTRACT

AN AGE HARDENABLE STAINLESS STEEL HAVING A WHOLLY AUSTENITIC STRUCTURE AT HOT WORKING TEMPERATURES FOR EFFICIENCY OF PROCESSING AND A WHOLLY MARTENITIC STRUCTURE AT THE TEMPERATURE OF FABRICATION AND USE FOR EFFECTIVENESS OF PRODUCT.

United States Patent 3,701,653 STAINLESS STEEL James L. Schanck, Beaver Falls, Pa., assignor to The Babcock & Wilcox Company, New York, NY. No Drawing. Filed Dec. 3, 1970, Ser. No. 94,994 Int. Cl. C22c 39/20 US. Cl. 75-124 4 Claims ABSTRACT OF THE DISCLOSURE An age hardenable stainless steel having a wholly austenitic structure at hot working temperatures for efficiency of processing and a wholly martenitic structure at the temperature of fabrication and use for effectiveness of product.

This invention relates to an improved stainless steel having age hardening characteristics.

More particularly, this invention relates to an age hardening stainless steel of improved composition possessing improved physical characteristics for ease in manufacture and enhancement of a product made therefrom.

In some cases, a stainless steel is an alloy of such things as iron, chromium and nickel. Occasionally, small amounts of certain other elements are added to enhance corrosion resistance and/or mechanical properties and, in some instances, to immunize the steel against the action of certain harmful environments. In regard to corrosion resistance, the chromium content seems to be the controlling variable and the effect of chromium may be enhanced by the addition of nickel. Also, the mechanical properties of the stainless steel, like other steels, are a function of the structure and composition of the steel itself.

The stainless steels, -by being available in a variety of structures, exhibit a range of mechanical properties which, when combined with their corrosion resistance, make these steels highly versatile from the standpoint of design. In general, a stainless steel having an austenitic structure exhibits the optimum in impact properties at low temperatures and an optimum in strength at elevated temperatures while such steels having a martensitic structure exhibit the optimum in the hardness at room temperature but are the most brittle of other types of steels at the ordinary temperatures of use.

The difiiculty with the art is that some specialized applications require a stainless steel which is not only corrosion resistant but which is also relatively hard and which possesses a relatively high impact strength at room temperature. These applications include such things as the manufacture of golf clubs where, in addition to the aforesaid properties, such stainless steels must have the strength to enable ease of fabrication at relatively high temperatures but with the ability to achieve the optimum in strength and hardness in the final product at the temperature of use.

In general, the austenitic steels which possess the desired properties at elevated temperatures are not hardenable to optimum by heat treatment but may be hardened to some extent by cold work. In the latter situation, for some grades, the austenitic structure is partially transformed to that of a martensitic type containing a minor amount of carbon. In any case, the degree of hardening achieved by such cold work is a function of composition. The disadvantage of the stainless steels having an austenitic structure is that work hardening is confined to those shapes that can be cold worked and in addition, it requires much more power in processing to actn'eve a satisfactory and acceptable degree of hardening in the finished prodnot.

The latter disadvantage is more or less overcome by an age hardening stainless steel which is essentially 16-17% chromium, 67% nickel composition containing titanium and aluminum. As would be expected, the degree and manner of hardening of such stainless steels is a function of the specific composition. At the relatively high temperatures of processing, the structure of such steels consists of both an austenite and a delta ferrite phase. On cooling, the austenite phase transforms to a low carbon martensite, so that at room temperature, the structure of this alloy consists of a martensite phase plus delta ferrite phase with some retained austensite. The age hardening occurs in both the martensite and ferrite phases but not in the austenitic phase. Aging in this case is accomplished by heating in the temperature range of 900 to 1050 F.

The practical difliculty of the conventional age hardening stainless steels lies in the physical characteristics they exhibit during the fabrication process at both cold and hot temperatures. It was found that while such an alloy could be extruded, the surface quality of the finished product left something to be desired. Also, unless excessive care was taken in the cold reduction and thermal treatment, the tubing for use as shafts would split during production. As a direct result, product yields were intolerably low even though standard operating procedures were precisely followed. It was further found that the impact strength of the item being produced in either the solution annealed or precipitation hardened state was minimal. The aforesaid were the specific difiiculties encountered in the manufacture of golf clubs.

The subject invention answers all the needs of the art, as heretofore described, with special reference to applications requiring a stainless steel possessing a propensity for ease of manufacture, at high temperatures, such as the forging and extruding operation, in combination with the achievement of the optimum in strength and hardness by the finished product at the ordinary temperatures of use.

It is therefore an object of this invention to provide an age hardening stainless steel having improved physical characteristics.

Another object is to provide a stainless steel of improved composition having age hardening characteristics.

A further object is to provide an age hardening stainless steel having a wholly martensitic structure at room temperature and enhanced physical properties at both room and elevated temperatures.

And still another object is to provide an age hardening stainless steel having a completely austenitic structure at annealing temperatures and a wholly martensitic structure at the temperature of use of the finished product.

A still further object is to provide an age hardening stainless steel having improved physical characteristics for ease of fabrication and enhancement of products made therefrom.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same become better understood by reference to the following detailed description.

In theory, the ability of stainless steels to be fabricated, formed or shaped with optimum efiiciency at elevated temperatures is achieved when such steel consists of essentially one phase. For instance, relatively small amounts of ferrite can be tolerated in the structure of the austenitic type of age hardenable stainless steel but must be kept within proper limits either by adjustment of the ultimate composition or by adjustment of each of the processing temperatures involved. The presence of delta ferrite in the microstructure of such austenitic stainless steel is extremely detrimental because, as a result, such steels exhibit extremely poor working characteristics at elevated temperatures. Again, in theory, the basis for these satisfactory performance characteristics of age hardening stainplasticity of the multiple phases of the microstructure. The ferrite structure is relatively soft while the structure of rite were present. The object of this invention as heretofore described, was to secure a microstructure which was wholly martensitic at room temperature after cooling from the annealing stage and the like. In order to expose any contamination of a ferrite structure that might be dethe austenite is relatively tough and the working of such 5 multiple phases at elevated temperatures causes rupture veloped in the heat at the high temperature of processing of the item undermanufacture. and present during the fabrication process, the specimens The subject invention is an age hardening stainless steel were, initially annealed at 2,200 F. for one hour and essentially possessing a structure having a single phase at water quenched prior to examination. Such a treatment both the elevated temperatures of processing and at the would produce a solid solution of all the elements present ordinary temperature encountered in the fabrication and and would show up any ferrite that might be present duruse of the finished product. Initially, at the annealing stage ing the hot working operation. It was found that alloys of processing, this stainless steel is wholly austenitic withhaving the aforesaid chemistry in the ranges specified were out any measurable amount of contamination in the strucwholly austenitic at annealing temperature and wholly ture- However, at room temperature, Such Steel has an martensitic at room temperature. As a result, such heats irreversible martensitic f p h' g the inherent aged to satisfactory levels of hardness without encounterage hardenable characteristic to such stainless steel. The i difficulty and possessed the optimum in impact strength, of ferrite can be tolerated in the structure of the austenltat room Emmi-aura type of FE hardenable fiamless Steel but must, be The following were heats, conditioned in accordance tranf?rmat1n h austemtl? Structure to one of "F with the process described, having the following physical Yerslb e martenslte is accomplished transformatlon characteristics based on the improved chemistry of this evel above room temperature of fabrication. As a result, a v the stainless steel of this invention does not contain fermventlon' rite in the microstructure which in considerable quantities .Heat 3079-3116 aforesald exammatlon mdlfiated can cause the aforesaid problemsinthe fabrication process ll? heat was compoi'ed of a wholly martensmc i and the finished product structure. The chromium content (15.38) was relatively The age hardening stainless steel of this invention in- 10W, as was the ferrite former content of alummum cludes the following ingredients by way of analysis in the The nickcl content 011 the other hand, Was ranges specified as expressed in percent by weight of the high enough to overcome ferrite forming tendencies of total alloy: these elements without promoting retained austenite.

Percent by weight Cr Ni Ti Al st 0 Mn s P g -?23 11 i; 3;2 W133 313333;: W133 tg iifiiii if???" It was found that the proper balancing of the ingredients of the above composition to secure a single-phase microstructure which is age hardenable to a wide range of strengths also enhanced the hot and cold working properties of the alloy and increased the resistance to impact loading at ordinary temperatures. It was found that an alloy having the specified constituents within the aforesaid ranges was completely austenitic in structure at the temperatures of hot working and possessed a transformation level above room temperature. This allowed the transformation of the structure, from austenitic to martensitic, to occur spontaneously upon cooling from the appropriate annealing temperatures. In addition, there is no austenitic phaseretained in the alloy to discourage precipitation by subsequent thermal treatments.

To obtain some latitude in the commercial processing of heats of a relatively large magnitude in volume, some broadening of the ranges was found to be expedient for such constituents as silicon, titanium and aluminum. Therefore, a preferred range for each of these ingredients, expressed as percent by weight, is as follows, viz:

A 25 ton electric furnace heat containing all the aforesaid ingredients, in the ranges specified, was melted and processed as an alloy without encountering any difficulty in either the steel or tube mill. Subsequent studies indicated improved mechanical properties were obtained with the heat of this improved chemistry, particularly in regard to impact strength at room temperature. The specimens were then forged to square bar stock and cut into smaller sections for metallographical study of the microstructure and aging characteristics. To be more specific, the specimens were examined metallograpically in the longitudinal direction to determine whether the microstructure was single phase or whether considerable amounts of fer- Heat 3261It was estimated by the aforesaid examination that this heat contained a microstructure which was entirely martensitic. The nickel content (7.94) was relatively high to balance the somewhat higher titanium (0.66) and aluminum contents (0.30), as well as What might be considered a mid-range chromium content (15.59). In addition the carbon content (0.034) is slightly higher than the previous heat.

Heat 3348-It was found that this heat had a completely martensitic microstructure. Even though the chromium (15.74), titanium (0.66) and aluminum (0.39)

contents are relatively high, the higher nickel (8.65) and carbon contents (0.05) balance them satisfactorily.

Heat 3426-The structure of the stainless steel made from this heat was wholly martensitic at room temperature. In this case, the strong ferrite formers, chromium (15.20) and titanium (0.42) were on the low side of the range specified for the present composition. This, coupled with the fact that nickel (7.85) lay to the high side of the range, raised the possibility that there would be some austenite in the structure and this would destroy the ability of the alloy to age harden.

As a further confirmation of the physical properties of the stainless steel of this invention, specimens from each of the above heats were cut, solution annealed at 1,600 F. for one hour and subsequently cooled in air. This is the solution annealing temperature ordinarily used in practice. Again, it was found that the structure achieved wasthat of a low carbon martensite. These were then'aged for various lengths of time from /2 hour to 15 hours. Aging temperatures were employed at 50 F. intervals between 850" F. and 1,200 F. The specimens were then air cooled from the aging temperature and hardness measurements were then taken.

In general, in the results obtained by the aforesaid procedure, the maximum hardness after aging was achieved by subjecting the specimen to temperatures of 850 F. Relatively little difference was noted among the specimensin the range of 800 to 900 F. However, a tendency towards overaging was exhibited by some specimens when held for longer periods of time at 950 F. Also, pronounced overaging was found to be induced with longer aging times at temperatures in the range of 1,000 F. to 1,200 F. At relatively higher temperatures, there appeared to be a tendency towards an increase of hardness as the time of aging progressed. In any event, hardness was increased in a dramatic fashion over that of the solution annealed condition within the space of /2 to 2 hours and the maximum hardness was achieved in about one hour. It is evident that the change of hardness would be easily controlled by a choice of temperature and times of aging treatment. Thus, the present alloy is readily versatile because various hardnesses will give different and reasonably predictable tensile strengths. However, if any retained austenite were present in the matrix, there would be a retardation of the age hardening process which would have been immediately detected.

As representative of the general nature of physical properties found for all the above heats, further teachings will be directed toward Heat 3426.

Hardness-After annealing at the conventional temperatures used in the mill, the hardness of the pieces was about Rockwell C, (R,,) 23-245. Essentially maximum hardness (R -44) was achieved by aging at 900 F. for eight hours. Only a minimal further increase (R -45) was secured by aging at this temperature for further seven hours duration. The indication is that aging at even lower temperatures, such as 850 F., would give higher hardness. Upon aging at higher temperatures, Such as 1,000 E, the hardness (R,,24) of the solution annealed condition rose in a dramatic fashion (R -40.5) and then either leveled 01f (R -37) or began to drop to a thoroughly overaged hardness condition such as (R -34) in the case of aging at 1050" F. and (R -3041) in the case of aging at 1100 F.

Tensile strength-The strength value of the pieces range from about 200,000 p.s.i., after aging to secure maximum hardness, down to 134,000 p.s.i. of the solution annealed material. This indicates that, as far as the tensile properties are concerned, the heat would be extremely versatile.

Impact strength.-Tests to determine this property were performed on finished machined pieces after cooling from the annealing temperature and aging with variable parameters being time and temperature. However, the control was merely solution annealed. The results are seen in Table I in which the aging treatment is correlated with impact strength and hardness.

TABLE I Aging Impact Hardtemperature, strength ness F. Time Cooling method (it-lbs.) R,

Control 1 145-140 25-27 ,100 1 hr Air cool 92-88 -31 1,100 8 hrs -do 97-96 28-29 1,000 3 hrs. min ..do 64-63 36-37 1,000 3 hrs. 35 min. Water quench.-- 62-55 35-36 1,000 8 hrs. Air cool 50-49 37-38 1,000 Water quench- 65-64 36-37 900. co 18-15 41-42 900-. Water queneh 21-21 -41 900 3 hrs Air cool 14-13 42-43 1 1,600 F, 1 hr., air cool.

It is seen that an appreciable degree of impact strength is retained by the pieces even at the higher levels of hardness. Thus, it has been shown that pieces from a heat of the improved analysis of this invention have relatively good impact properties.

In summary, all of the above heats upon analysis were wholly austenitic in microstructure at the elevated temperatures of the annealing stage but entirely martensitic upon cooling to room temperature whether this latter stage was carried out in air or water. This variation in microstructure at extremes of temperature in the processing of an alloy such as this stainless steel has been found to be highly desirable. As a result, a stainless steel alloy was obtained which exhibited ease of workability at the elevated temperatures of the fabrication process while possessing the optimum in impact strength and workability at room temperature and, in addition, the ability to be age hardened by subsequent thermal treatments which of themselves are relatively simple. The age hardened product of such treatment would still possess a relatively good impact strength. Upon the basis of this concept, the presence of a wholly martensitic structure at room temperature, after cooling or quenching from elevated temperatures, would be prima facie evidence of enhanced workability and ease of fabrication in the mill and a strong indication of the ability to achieve the optimum in physical properties by the finished product especially impact strength and hardness.

What is claimed is:

1. An improved age hardening stainless steel having a wholly martensitic microstructure at room temperature consisting essentially of:

(a) Chromium from 15.2 to 16.25 percent,

(b) Nickel from 6.8 to 8 percent,

(c) Titanium from 0.4 to 0.7 percent,

((1) Aluminum from .3 to .45 percent,

(e) Silicon up to 0.5 percent,

(f) Manganese from 0.7 to 0.9 percent,

(g) Sulfur up to 0.025 percent,

(h) Phosphorous up to 0.025 percent,

(i) Carbon up to 0.04, all percentages being by Weight,

and

(j) the balance being iron.

2. The improved age hardening steel of claim 1 consisting essentially of:

(a) Chromium at 15.75 percent,

(b) Nickel at 7.5 percent,

(0) Titanium at 0.6 percent,

(d) Aluminum at 0.4 percent,

(e) Silicon at 0.45 percent,

(f) Manganese at 0.80 percent,

(g) Sulfur up to 0.025 percent,

(h) Phosphorous up to 0.025 percent,

(i) Carbon at 0.035 percent, all percentages being by weight, and the balance being iron.

3. The age hardening stainless steel of claim 1 consisting essentially of:

(a) from 0.3 to 0.5 percent silicon;

(b) from 0.55 to 0.7 percent titanium;

(c) from 0.3 to 0.4 percent aluminum, all said percentages being by weight.

4. The age hardening stainless steel of claim 3 consisting essentially of:

(a) 0.4 percent silicon,

(b) 0.63 percent titanium, and

(c) 0.35 percent aluminum, all said percentages being by weight.

References Cited UNITED STATES PATENTS 1,538,337 5/1925 Kuehn -128 T 1,538,360 5/1925 Smith 75-128 T 2,958,617 11/1960 Perry 75-128 T HY LAND BIZOT, Primary Examiner US. Cl. X.R. 

